Chapters 24 & 25

The Digestive System, Nutrition & Body Metabolism

The Digestive System & Body MetabolismThe Digestive System & Body Metabolism

Digestion

Breakdown of ingested food

Absorption Passage of nutrients into the blood

Metabolism Production of cellular energy (ATP)

Elimination

Categories of DigestionCategories of Digestion Mechanical digestion

Mixing of food in the mouth by the tongue

Churning of food in the stomach

Segmentation in the small intestine

Chemical Digestion

Enzymes break down food molecules into their building blocks

Each major food group uses different enzymes

Carbohydrates are broken to simple sugars

Proteins are broken to amino acids

Fats are broken to fatty acids and alcohols

NutritionNutrition Nutrient – substance used by the body for growth,

maintenance, and repair

Nutrients are considered essential if they cannot be produced by the body and must be obtained through diet

Categories of nutrients

Carbohydrates: simple sugars, starches, fiber

Lipids: triglycerides, phospholipids, fatty acids

Proteins: amino acids

Vitamins

Mineral

Water

Organs of the Digestive SystemOrgans of the Digestive System

Two main groups

Alimentary canal: continuous coiled hollow tube; pathway that food takes from mouth to anus

Accessory digestive organs: help the organs of the alimentary canal carry out their digestive role

Processes of the Digestive SystemProcesses of the Digestive System Ingestion: getting food into the mouth

Propulsion: moving foods from one region of the digestive system to another

Peristalsis: alternating waves of contraction

Segmentation: moving materials back and forth to aid in mixing

Mechanical Digestion: physically preparing food for breakdown

Chemical Digestion: nutrient molecules are broken down into smaller absorbable building blocks (protein to AA)

Absorption

End products of digestion are absorbed in the blood or lymph

Food must enter mucosal cells and then into blood or lymph capillaries

Defecation

Elimination of indigestible substances as feces

Processes of the Digestive SystemProcesses of the Digestive System

Control of Digestive ActivityControl of Digestive Activity Mostly controlled by reflexes via the

parasympathetic division

Chemical and mechanical receptors are located in organ walls that trigger reflexes

Stimuli include: Stretch of the organ

pH of the contents

Presence of breakdown products

Reflexes include: Activation or inhibition of glandular secretions

Smooth muscle activity

Body Energy BalanceBody Energy Balance

Energy intake = total energy output

(heat + work + energy storage)

Energy intake is liberated during food oxidation

Energy output

Heat is usually about 60%

Storage energy is in the form of fat or glycogen

Organs of the Digestive SystemOrgans of the Digestive System

Layers of Alimentary Canal Organs ILayers of Alimentary Canal Organs I

Mucosa

Innermost layer

Moist membrane

Surface epithelium

Small amount of connective tissue (lamina propria)

Small smooth muscle layer

Layers of Alimentary Canal Organs IILayers of Alimentary Canal Organs II Submucosa

Just beneath the mucosa

Soft connective tissue with blood vessels, nerve endings, and lymphatics

Muscularis externa – smooth muscle

Inner circular layer

Outer longitudinal layer

Serosa

Outermost layer – visceral peritoneum

Layer of serous fluid-producing cells

Layers of Alimentary Canal OrgansLayers of Alimentary Canal Organs

Organs of the Alimentary CanalOrgans of the Alimentary Canal

Mouth

Pharynx

Esophagus

Stomach

Small intestine

Large intestine

AnusDigestion Animation

Processes of the MouthProcesses of the Mouth Mastication (chewing) of

food

Mixing masticated food with saliva

Saliva contains the digestive enzyme amylase which begins the breakdown of starch to sugar

Initiation of swallowing by the tongue

Allowing for the sense of taste

Pharynx FunctionPharynx Function

Serves as a passageway for air and food

Food is propelled to the esophagus by two muscle layers Longitudinal outer layer

Circular inner layer

Food movement is by alternating contractions of the muscle layers (peristalsis)

EsophagusEsophagus Runs from pharynx to

stomach through the diaphragm

Conducts food by peristalsis (slow rhythmic squeezing)

Passageway for food only (respiratory system branches off after the pharynx)

Stomach AnatomyStomach Anatomy Located on the left side of the abdominal cavity

Food enters at the cardioesophageal sphincter

Regions of the stomach Cardiac region – near the heart

Fundus

Body

Phylorus – funnel-shaped terminal end

Food empties into the small intestine at the pyloric sphincter

Stomach AnatomyStomach Anatomy Rugae: internal folds of the mucosa

External regions

Lesser curvature

Greater curvature

Layers of peritoneum attached to the stomach

Lesser omentum: attaches the liver to the lesser curvature

Greater omentum: attaches the greater curvature to the posterior body wall

Contains fat to insulate, cushion, and protect abdominal organs

Stomach AnatomyStomach Anatomy

Figure 14.4a

Stomach FunctionsStomach Functions

Acts as a storage tank for food

Site of food breakdown

Chemical breakdown of protein begins

Delivers chyme (processed food) to the small intestine

Specialized Mucosa of the StomachSpecialized Mucosa of the Stomach Simple columnar epithelium, composed entirely of goblet

cells, which produce a protective coat of alkaline mucus

Mucous neck cells: produce a sticky acidic mucus different from that of goblet cells. Exact function unknown.

Chief cells: produce inactive protein-digesting enzymes called pepsinogens

Parietal cells: produce hydrochloric acid that activate and provide optimal conditions for pespin

Endocrine cells: release a variety of hormones/hormone-like products including gastrin, which stimulates HCl secretion

Structure of the Stomach MucosaStructure of the Stomach Mucosa Gastric pits formed by folded mucosa

Glands and specialized cells are in the gastric gland region

Small IntestineSmall Intestine

The body’s major digestive organ

Site of nutrient absorption into the blood

Muscular tube extending form the pyloric sphincter to the ileocecal valve

Suspended from the posterior abdominal wall by the mesentery

Subdivisions of the Small IntestineSubdivisions of the Small Intestine

Duodenum Attached to the stomach

Curves around the head of the pancreas

Jejunum Attaches anteriorly to the

duodenum

Ileum Extends from jejunum to large

intestine

Villi & Microvilli of the Small IntestineVilli & Microvilli of the Small Intestine Villi

Fingerlike structures formed by the mucosa

Give the small intestine more surface area

Microvilli

Small projections of the plasma membrane

Found on absorptive cells

Structures Involved in Absorption of NutrientsStructures Involved in Absorption of Nutrients

Absorptive cells

Blood capillaries

Lacteals (specialized lymphatic capillaries)

Figure 14.7b

Folds of the Small IntestineFolds of the Small Intestine Called circular folds or

plicae circulares

Deep folds of the mucosa and submucosa

Do not disappear when filled with food

The submucosa has Peyer’s patches (collections of lymphatic tissue)

Digestion in the Small Intestine IDigestion in the Small Intestine I Source of enzymes that are mixed with chyme

Intestinal cells

Pancreas

Bile enters from the gall bladder – helps emulsify fats

Digestion in the Small Intestine IIDigestion in the Small Intestine II Enzymes from the brush border

Break double sugars into simple sugars

Complete some protein digestion

Pancreatic enzymes play the major digestive function Help complete digestion of starch (pancreatic

amylase)

Carry out about half of all protein digestion (trypsin, pepsin etc.)

Responsible for fat digestion (lipase)

Digest nucleic acids (nucleases)

Alkaline content neutralizes acidic chyme

Propulsion in the Small IntestinePropulsion in the Small Intestine Peristalsis is the major means of moving food

Segmental movements

Mix chyme with digestive juices

Aid in propelling food

Absorption in the Small IntestineAbsorption in the Small Intestine

Water is absorbed along the length of the small intestine

End products of digestion

Most substances are absorbed by active transport through cell membranes

Lipids are absorbed by diffusion

Substances are transported to the liver by the hepatic portal vein or lymph

Large IntestineLarge Intestine Larger in diameter, but shorter than the small intestine

Frames the internal abdomen

Functions of the Large IntestineFunctions of the Large Intestine

Absorption of water

Eliminates indigestible food from the body as feces

Does not participate in digestion of food

Goblet cells produce mucus to act as a lubricant

Food Breakdown and Absorption in the L. I.Food Breakdown and Absorption in the L. I.

No digestive enzymes are produced

Resident bacteria digest remaining nutrients Produce some vitamin K and B

Release gases

Water and vitamins K and B are absorbed

Remaining materials are eliminated via feces

Structures of the Large IntestineStructures of the Large Intestine

Cecum – saclike first part of the large intestine

Appendix

Accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis)

Hangs from the cecum

Structures of the Large IntestineStructures of the Large Intestine

Colon Ascendin

Transverse

Descending

S-shaped sigmoidal

Rectum Anus – external body opening

Propulsion in the Large IntestinePropulsion in the Large Intestine Sluggish peristalsis

Mass movements Slow, powerful movements

Occur three to four times per day

Presence of feces in the rectum causes a defecation reflex Internal anal sphincter is relaxed

Defecation occurs with relaxation of the voluntary (external) anal sphincter

Accessory Digestive OrgansAccessory Digestive Organs

Salivary glands

Teeth

Pancreas

Liver

Gall bladder

Salivary GlandsSalivary Glands Extrinsic salivary glands: activated by entry of

food into the mouth (can also be triggered by sight/smell of food)

Parotid glands – located anterior to ears

Submandibular glands

Sublingual glands

Small intrinsic salviary (buccal glands): scattered throughout the oral mucosa constantly secrete small amounts of saliva to maintain mouth moisture

SalivaSaliva

Mixture of mucus and serous fluids

Cleanses the mouth

Moistens food and helps compact it to form a food bolus

Contains salivary amylase to begin starch digestion

Dissolves chemicals in food so they can be tasted

TeethTeeth The role is to masticate (chew) food

Humans have two sets of teeth

Deciduous (baby or milk) teeth

20 teeth are fully formed by age two

Permanent teeth

Replace deciduous teeth beginning between the ages of 6 to 12

A full set is 32 teeth, but some people do not have wisdom teeth

Classification of TeethClassification of Teeth

Incisors: chisel-shaped for cutting or nipping of pieces of food

Canines (cuspids/eyeteeth): conical or fang-like for tearing and piercing

Premolars (bicuspids) & Molars: broad crowns with rounded cusps (tips) suited for grinding and crushing.

Classification of TeethClassification of Teeth

Regions of a Tooth IRegions of a Tooth I Crown – exposed part

Outer enamel

Dentin

Pulp cavity

Neck Region in contact with

the gum

Connects crown to root

Regions of a Tooth IIRegions of a Tooth II

Root

Periodontal membrane attached to the bone

Root canal carrying blood vessels and nerves

PancreasPancreas Produces a wide spectrum of digestive enzymes

that break down all categories of food

Enzymes are secreted into the duodenum

Alkaline fluid introduced with enzymes neutralizes acidic chyme

Acini: clusters of secretory cells around ducts

Endocrine products of pancreas secreted from pancreatics Isles of Langerhans

Insulin – lower blood glucose by conversion to glycogen

Glucagon – raise blood sugar by breakdown of glycogen

LiverLiver

Largest gland in the body

Located on the right side of the body under the diaphragm

Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament

Connected to the gall bladder via the common hepatic duct

Role of the Liver in MetabolismRole of the Liver in Metabolism

Several roles in digestion – primarily to produce bile

Detoxifies drugs and alcohol

Degrades hormones

Produce cholesterol and blood proteins (albumin and clotting proteins)

BileBile Yellow-green alkaline solution produced by cells

in the liver

Emulsifies fats into millions of small fatty droplets to provide large surface area for fat-digesting enzymes (lipase) to work on

Composition Bile salts – primary emulsifying agents

Bile pigment (mostly bilirubin waste product of hemoglobin from breakdown of worn erythrocytes)

Cholesterol

Phospholipids

Electrolytes

Gall BladderGall Bladder Sac found in hollow fossa of liver

Stores bile from the liver by way of the cystic duct

Concentrates bile to 10x that of secretion from liver by absorbing some of its water and ions

Bile is introduced into the duodenum in the presence of fatty food

Gallstones can cause blockages

Crystallization of cholesterol due to overabundance or too little bile salts

Sharp crystals cause pain when gallbladder contracts